Water Quality Explained: How It Can Affect Your Axolotl's Health
World Small Animal Veterinary Association World Congress Proceedings, 2015
R. Loh, BSc, BVMS, MPhil, MANZCVS, CertAqV
The Fish Vet, Perth, WA, Australia


Axolotls live in intimate contact with their watery environment. Moreover, they rely on the qualities of water for many biological processes including respiration, nutrition, hydration, excretion and more. It is no wonder then that all persons dealing with aquatic animal health always stress on the importance of water quality.

It is advisable to test the water quality as the first course of action when investigating the reasons for disease in axolotls. This can help you quickly identify issues with water if it is the case. It will also allow us to modify their course of action or treatment plan.

There is a variety of water test kits available and they range from liquid drops or test strips, with visual colorimetric changes, to more expensive digital probes. The choice depends on budgetary constraints and the degree of accuracy required. For the general veterinary clinic, many of the aquarium test kits would be suitable for testing all the important parameters.

The critical water quality parameters that directly affect the axolotl's health include water temperature, ammonia (NH3), nitrite (NO2-), nitrate (NO3-), pH, carbonate hardness (KH, also known as alkalinity), general hardness (GH, also known as permanent hardness) and dissolved oxygen (DO). Additionally, some parameters have diurnal fluctuations. Imbalances in any one of these parameters may have an impact on others and these inter-relationships will be explained. But before the specifics of each water quality parameter is discussed, they must understand the nitrogen cycle.

Nitrogen-Cycle and Bio-Filters

Axolotls continually produce ammonia as their waste product and at high levels, can be toxic. The biofilter provides a physical substrate for beneficial bacteria to colonise so they can detoxify wastes. The conversion of ammonia to nitrite (predominantly by Nitrosomonas, Nitrospira and Nitrosococcus), and from nitrite to nitrate (predominantly by Nitrobacter, Nitrococcus and Nitrospina) is termed the "nitrogen cycle." In the natural environment, the end product (nitrate) will be incorporated into plants/algae. However, in an aquarium without plants, nitrate will accumulate unless it is removed by partial water changes or by using a denitrification unit.

Several factors affect the performance of the biofilter and they include:

 Temperature (works between 12–58°C, most efficient between 28–36°C)

 The pH of the water (reactions occurring faster between a pH of 8–9, but is impeded at pH < 5)

An established, well-balanced aquarium should consistently have zero ammonia and nitrite. If ammonia and/or nitrite are allowed to build up considerably, deaths result; a condition called, "new tank syndrome."

Some chemicals used in the treatment of diseases may harm the biofilter. They include most antibiotics and disinfectants like methylene blue. It is advisable to monitor levels of ammonia and nitrite following treatment, until the biofilter is re-established.


Axolotls are particularly sensitive to temperature changes because they are poikilotherms, and water is such an excellent thermal conductor. Water temperature affects their metabolism (metabolic rate), feed intake, growth, reproduction, physiological processes (affects the function of enzymes), disease immunity and general activity.

Axolotls are a cool water species, and where they occurred naturally in the wild, their habitat was filled by springs and melting water from snow-capped mountains. Their ideal environment will be in the vicinity of 15–18°C. If the water temperature rises and maintains above 24°C (e.g., during hot spells in summer), axolotls will present with clinical signs ranging from inappetance, ascites and uncontrollable floating. Emergency treatment by placing the axolotl in a dish in the refrigerator is helpful. Consider antibiotic therapy to prevent secondary bacterial infections. Water temperature can be maintained by the use of thermostatically-controlled chillers.


The pH of the water is a measurement of its acidity, where values above 7.0 are alkaline and values below this are acidic. Numerous biological processes depend on pH and there are species differences for pH requirements.

Axolotls held in environments that are too acidic have difficulty maintaining physiological functions. Clinical signs displayed by axolotls kept in water at a pH of 4.5 for example, include excess mucus production, inappetance, listlessness, floating, ascites, and death. Remember also that biofilter activity is inhibited at pH levels below 5.0, and cases may be complicated by new tank syndrome.

Poor tank hygiene (accumulation of organic material) is often the cause of low pH. In such cases, water changes with the removal of organic matter and the addition of sodium bicarbonate (NaHCO3 [baking soda]), limestone, crushed coral or shells are recommended. There are also commercial pH buffers available at the local fish/pet store. Increases in pH should be made gradually, with no more than 0.5 units per day.


Ammonia (NH3) is a major waste product of axolotls. It is a strong cell poison and can cause damage to the gills, impairing gas exchange and neurological damage. In water, it can be present in two forms; highly soluble toxic unionised ammonia (NH3, also known as free ammonia nitrogen or FAN), or the less toxic ammonium ion (NH4+) - the sum of which is known as TAN (total ammonia nitrogen). The proportion of toxic ammonia (FAN) is influenced by several factors; it is higher with increasing pH and temperature and with decreasing salinity. Of these, the pH of water is the most crucial factor. Water test kits usually measure the TAN. You can determine whether toxic levels of FAN are present with the help of a chart.

The most common causes of ammonia spikes include:

 Increased feeding rate.

 Increased stocking density.

 Damage to biofilter (pump has stopped for a significant length of time, filter becomes clogged, filter is washed too thoroughly, chemicals including antibiotics used).

Treatment for ammonia toxicosis involves performing multiple partial water changes (25–50% each time), using chemical filtration (e.g., zeolite), adding proprietary ammonia-binders and lowering the pH towards 6.5. Owners can also supplement the biofilter with more nitrifying bacteria which are available from fish shops. Feeding should be minimised and only gradually re-introduced. If the ammonia levels are extremely high and is not expected to be controlled within 24 hours, then axolotls should be temporarily housed in the refrigerator (at 8°C) while the aquarium cycles (empty aquarium still needs to be supplied food).


Nitrite (NO2-) is generated from the oxidation of ammonia by nitrifying bacteria. Elevated levels often occur during the early stages of setting up new aquariums as the biofilter undergoes the nitrogen cycle process. A sudden spike in the nitrite usually means there is an imbalance in the system.

Nitrite poses a risk at levels above 0.5 mg/L, and can be lethal when it exceeds 2 mg/L. The reasons for nitrite spikes are similar to those for ammonia. They could stem from something as simple as washing the biological filter media too thoroughly (Nitrobacter are not as adherent, and may be washed away). This means that the ammonia can continue to be converted to nitrite, but the rate of conversion of nitrite to nitrate will be much reduced and hence the nitrite spikes.

The relationship between total nitrite and unionised forms such as nitrous acid (HNO2) is inversely pH dependent. Both nitrite and nitrous acid are toxic, but nitrous acid is more harmful. This explains why nitrite is more toxic in acidic water. It is also more toxic in soft water, at higher temperatures and in lower salinity.

Nitrite toxicosis is addressed by withdrawing food, followed by multiple, large, partial water changes (25–50%). Methylene blue should be added at a rate of 1–2 mg/L to reverse the process of methaemoglobin formation (predilute the chemical, and avoid the biofilter intake). Salinity (NaCl) should be increased by up to 2 mg/L, to competitively inhibit nitrite uptake by the gills. The pH of the water can be gradually raised to 8.5, and the water temperature reduced (e.g., to 5°C), to minimise the proportion of the more toxic form of nitrite. Provide supplemental aeration, and additional nitrifying bacteria for the biofilter. Mortalities can still be expected from the damage.

If the nitrite levels are extremely high and is not expected to be controlled within 24 hours, then axolotls should be temporarily housed in the refrigerator (at 8°C) while the aquarium continues to cycle (empty aquarium still needs to be supplied food).


Nitrate is the end-product of organic and inorganic decay and it accumulates in tanks over time (high levels indicate poor husbandry), especially those without plants. Nitrate level should be kept below 50 mg/L. High levels of nitrate are an indicator of poor husbandry. By extrapolating from fish health, high levels of nitrate is suggested to cause depressed immunity. In axolotls, exophthalmia and corneal opacity can be a sign of prolonged exposure to high levels of nitrate.

Additionally, poor husbandry can indirectly lead to depletion of alkalinity and lead to acidic water. Axolotls would develop a diffuse film of mucus and present ill. Again, management would involve a combination of multiple water changes and addressing the underlying cause(s).

Carbonate Hardness (KH) or Alkalinity

This is the measurement of the capacity for water to neutralise an acid (i.e., the buffering capacity against pH crashes). Most aquarium KH test kits measure total alkalinity (or total pH buffering capacity). The alkalinity is primarily composed of bicarbonate ions (HCO3-) and carbonate ions (CO32-), hence the common name of "KH". However, it may also consist of diphosphates (H2PO4-), sulfates and borates. KH is essential to stabilise the pH of water, it is an important source of energy for nitrifying bacteria and it is used by plants for photosynthesis (when carbon dioxide is absent).

Most aquarium test kits measure "total alkalinity" rather than just the carbonates. The desired KH axolotls is 3–8°. Alkalinity can be increased by the addition of salts of carbonates, hydroxides, and the speed and stability of effect is influenced by their solubility (pKa value).

General Hardness (GH)

This is a measurement of all chemically bivalent cations (primarily comprising calcium and magnesium) that in nature, is related to the geology of the water source. The units of measurement can be expressed in several ways, where: 1° = 17.8 mg/L = 17.8 ppm.

Typically, water is described as 'hard' if the GH is > 16°, and 'soft' if it is < 8°. Hard water is noticeable because they cause scaling problems with plumbing/pipework and fittings. Since the axolotl's natural environment is supplied from springs and mountain snowmelt, they require moderately-hard water ranging from 7–14°.


Oxygen is vital for a healthy aquarium. The maximum oxygen carrying capacity of water is greatly affected by the temperature, where dissolved gases are reduced at higher temperatures. Healthy systems should be 70–100% saturated, and contain 10 mg/L at 15°C.

Excess oxygen saturation of > 100% have been implicated in gas bubble disease in fish, and are likely to occur in axolotls. The lower requirement for oxygen by axolotls are not published, possibly because they can cope taking surface air into their lungs.


Chlorine (Cl2) is commonly found in tap water at a rate of 0.5–2.0 mg/L and these levels are harmful to axolotls. Testing for chlorine level is not always possible or necessary because a diagnosis can often be made from the clinical history. Treatment involves the addition 7.4 mg/L sodium thiosulphate (chlorine neutraliser) to every 1 mg/L chlorine that is present in the water. Since the neutralising reaction consumes oxygen and lowers the pH, vigorous aeration and buffering with sodium bicarbonate is recommended.


It is important to understand the fundamentals of water quality, how it affects axolotl health, their relationship with other water parameters and how it influences the choice of treatment. Water quality testing is indispensable and must be conducted with every axolotl disease investigation. If there are problems with the water, then consider performing water changes, chemical filtration, withholding feed and temporary housing of axolotls in the refrigerator. Additional actions will depend on the interrelationships that the aberrant parameter has with water quality values. Water quality parameters for axolotls are summarised in the table below.



Optimal range

Tolerance range



> 50

> 10

Water temp







< 2.0




< 0.5



< 10

< 110













Oxygen saturation







< 15


1.  Ambystoma. Sal-Site. www.ambystoma.org. 2015.

2.  Bjorklund NK, Duhon ST. The Mexican axolotl as a pet and a laboratory animal. In: Ackerman L, ed. Biology, Husbandry and Health Care of Reptiles and Amphibians. Jersey City, NJ: Tropical Fish Hobbyist; 1997.

3.  Clare JP. Axolotls. www.axolotl.org. 1999.

4.  Loh R, Landos M. Fish Vetting Essentials. Perth, Australia: Richmond Loh Publishing; 2011.

5.  Loh R. Fish Vetting Axolotls. Perth, Australia: Richmond Loh Publishing; (in-prep).


Speaker Information
(click the speaker's name to view other papers and abstracts submitted by this speaker)

R. Loh, BSc, BVMS, MPhil, MANZCVS, CertAqV
The Fish Vet
Perth, WA, Australia

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